The weather outside is far from frightful. Fifty degrees today, can you believe it? Abundant sunshine at least through noon should allow most spots today to see & exceed the big 5-0! Look for light, slightly variable, Southerly winds as well. I'd still suggest a windbreaker since we'll be recovering from chilly low temperatures this morning.

Photo by Silvia S. of Ermesinde, Portugal

Holiday Travel Conditions:or even for those staying Districted...

The Capital Weather team will keep you apprised on the rain to fall on Sunday. Even if it doesn't look too heavy it will dampen holiday commuting through early afternoon over most of the area. Watch for wet interstates 95 & 81 through the midsection of the East Coast. If staying around DC to view the National Christmas Treeor Congressional Christmas Tree, try viewing late on the evening of December 24. You should be dry if venturing out to those two sites or perhaps the National Menorah, which is to be lit on the following evening (December 25) well after the rain likely tapers. Watch for chilly, breezy conditions, too.

Expanding on last week's topic of the Gulf Stream and U.S. East Coast cyclogenesis, today I'll explore the broader ocean circulation machine. We won't talk about temperature gradients in the atmosphere, but rather how the ocean helps to moderate & "smooth"average temperature swings in the atmosphere. It is through ocean currents, that the extremes in incoming solar energy Earth experiences during the tilt from hemispheric summer versus winter, are mitigated.

"Why do the coldest average temperatures occur in January instead of focused directly around the December 21 Solstice--when the northern hemisphere receives the least amount of sunlight?"

Ocean currents flow in complex patterns as determined by wind, the water's salinity, heat content, bottom topography, and the Earth's rotation. Currents are present in every basin around the globe and they move large amounts of water on a very broad scale. The scale magnitude we're talking? Roughly one gallon of ocean water being jettisoned around the globe for every dollar comprising the yearly U.S. National Budget!

When we start looking at lower, tropical latitudes, we notice very warm water (see graphic at left). Generally warmer water stays nearer the surface of oceans since it is less dense. This shallower layer of warm, buoyant water on any ocean's surface, is thus highly driven by the average motion of surface winds.

The next component of the "Ocean conveyor" occurs as water cools near the poles. Propulsion at higher, colder latitudes is a whole different matter. In the North Atlantic, for instance, the dominant feature is the sinking of cold, salty (and therefore denser) waters. In essence, this creates a void that pulls warm, salty surface waters northward--reinforcing the aforementioned wind motions acting on ocean water at low, tropical latitudes.

The sinking motion toward the bottom of ocean creates cold, deep-running water currents. This increases the importance of channels available in theocean floor topography as well as the level of fresh water glacial melting might inject into ocean waters at high latitudes (remember density & salinity drive the cold portion of ocean currents). As these cold mixtures of deep, dense water circulate back toward the tropics,water warms, rises, and is driven westward & northward (in the North Atlantic basin) to complete the wind-driven portion of the process.

Heat is disbursed along the way from the waters to the atmosphere via simple atmospheric contact with the water and wind-driven, convective motion of air. In last week's Gulf Stream example, right here--near us in the North Atlantic--large quantities of heat transfer to the atmosphere, and prevailing winds carry this relinquished heat eastward to warm Europe. Due to the rotation of the Earth, currents like the Gulf Stream are deflected to the right (when observing from space) in the northern hemisphere and to the left in the southern hemisphere. This "Coriolis force" you might think only applies to AIR in-motion. However, one must think of the atmosphere as a giant "ocean of air" and remember that rotationally-deflected surface wind--along with the smaller rotational response of the ocean waters--impact the motion of warm water rising to higher latitudes from the tropical regions.

There is a time-delay between shortest average day length--Winter Solstice is when a hemisphere receives its smallest amount of solar radiation--and coldest average temperature. This complex oceanic system, even when not in motion circulating, stores heat. The average temperature of ocean waters relatively stays much more constant than air temperature. This is due to liquid water's high specific heat. When moving above the molecular scale, a standalone property of any ocean is its intrinsic "heat sink" (a system which can absorb obscene amounts of heat without changing its own physical properties) nature. The presence of all this water absorbs the largest portion of Earth's incoming solar radiation. Think about it: combined, the World Ocean covers 70% of the Earth's surface and would cover our entire oblate spheroid in a 2-mile-thick layer of liquid water! The world's ocean water truly dictates the distribution of heat through its conveyor-belt motion of water. The atmosphere takes time to reach its warmest (in summer) and coldest (in winter) because of our vast oceans' abilities to store and gradually release heat from the Sun's radiation weeks prior.

Simultaneously reaching a critical "tipping point" (but on a smaller scale) around the start of Astronomical Winter is continental snowpack buildup in Canada and the northern United States. Once this vast area of continental land is blanketed in snow cover, two things begin to happen. One, natural terrestrial, long-wave (IR) radiation is cut-off from warming the atmosphere just above the ground. Remember snow is a great insulator and barrier both from incoming radiation above its coverage area AND infrared trying to escape from the ground underneath (more in comments section). Two, air masses advancing south from the North Pole are able to plummet into the southern U.S. without any warming modification via ground-to-air conduction processes. Each Jet Stream trough that successively develops over the North American Continent throughout a Winter season, has access to colder and colder air--reaching average coldest temperatures the second week of January throughout the hemisphere. Successful intrusions of unmodified air masses from the northern Arctic regions that possess cold domes of high pressure sitting over permafrost (glaciers or snowpack) create a positive feedback loop. The solar minimum allowing for formation of this feedback loop, it isn't until AFTER the conditions have established themselves that the chilly, physical manifestations are felt at our distance from the North Pole.

At D.C. National, the coldest average maximum temperature is 42 degrees, while coldest average minimum temperature is 27. The timeframe for these coldest temperatures is Jan. 9-23.

Snow Lover's Crystal Ball

Next Chance of Accumulating Snow: Middle of the upcoming weekProbability: 15%Potential Impact:Commentary: Fewer signs exist today that the atmospheric pattern next week may bring flakes to DC. Snow showers are much more likely in the far western portions of our region, especially with NW winds behind a Clipper system.

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This site is the archive for capitalweather.com, a website specializing in Washington DC weather, forecast, and climate information, actively maintained from 2004 through 2008. In 2008, we moved to the Washington Post and continued work as the Capital Weather Gang. You can find links to our current incarnation below. Thanks for browsing.